I have three computers, A, B, and C, and all are dual-homed. All six ports are on a 100Base-T router. Computers A and B need to talk to C (A and B don't need to talk directly). Each interface is a 1000Base-T interface, so I would like to force A and B talk to C via a new, third interface on each machine, for the faster network connection. I thought about using a crossover cable for this, but I'm not too sure if that's the best way of doing things. Buying a new switch seems like overkill, but it may be the best way to go. Any thoughts or feedback is most welcome. Thanks.

there's no need for another switch, assuming it too is a gigabit switch in line with the new cards. your best bet is probably a seperate subnet to put these cards on for logical seperation, and that would then take care of a lot of the other ugly things you might otherwise edge towards, like packet marking etc... if the gig's are all on a 192.168.123.0/24 subnet or whatever then if you connect to 192.1687.123.C from machine A, you're have no option but to use the gig nics. also as there is no routing going on, return routes will be no issue either. don't worry too much about connecting them to the same switch, it's going to be oblivious to the traffic on it, unless it's a higher end thing, in which case,m a seperate vlan for it is even better. it's actually simpler using seperate networks too as if all nics were on a single subnet you can often have the nic containing the default route answering ARP requests for the other cards, so whilst the gig nic as a different IP to the 100m one, the slower card may still say it's got the gig's ip. with seperate subnets that won't happen.